The invention relates to an apparatus for encapsulating with plastic lead frame that carries at least one chip, comprising mutually movable mould halves which in the closed position bound a mould cavity for receiving a lead frame, a feed runner connecting onto the mould cavity for supplying encapsulating material and a plunger-cylinder connected onto this feed runner for carrying up encapsulating material under pressure. Such an apparatus is known from the European patent application 89.203003.2.
The problem with apparatus of this type is that despite a close fitting of the plunger in the cylinder there is the danger of liquid encapsulating material running along the wall of the plunger. The cylinder wall is hereby fouled, which results in interruptions in the encapsulating process.